Glass sheet processing equipment has previously utilized roller conveyors for conveying glass sheets horizontally during the particular type of processing to be performed. Such roller conveyors have previously been utilized with annealing lehrs, tempering furnaces, and quench units for tempering systems used to process flat glass as well as for heating furnaces used to heat glass sheets in preparation for bending with or without subsequent tempering. Until about ten years ago, glass sheet roller conveyors were almost all driven by chains or gears which necessitated the use of bearings for supporting the opposite ends of the conveyor rolls. Introduction of a glass sheet roller conveyor as disclosed by the patents discussed below provided frictional driving of the conveyor rolls and also eliminated the necessity for using journals to support the rolls as was necessary with prior glass sheet conveyors.
U.S. Pat. No. 3,806,312 discloses a glass sheet tempering system whose furnace includes a horizontal roller conveyor having a friction drive mechanism for driving rolls of the conveyor in order to provide conveyance of glass sheets over the rolls. The rolls of the conveyor span the gap between a pair of spaced horizontally extending surfaces over which driving reaches of a pair of continuous drive loops are respectively driven to support and frictionally drive the ends of the rolls. Both support surfaces are located within the heating chamber of the furnace and the drive loops utilized are solid steel belts which can withstand the high temperature present upon passage through the heating chamber. During operation of this drive mechanism, the driving reaches of the continuous drive loops are maintained taut so as to maintain the upper surfaces of the rolls in a single plane along which the glass sheets are conveyed over the rolls. Also, a coupling between a pair of drive sheaves that respectively drive the continuous pair of drive loops at one end of the system and adjustable supports for each of the sheaves provides coordinated driving thereof in a manner that ensures conveyance of the glass sheets in the direction intended with minimal lateral drift. Upon commercial introduction into the glass tempering industry about 10 years ago, this friction drive mechanism was accorded immediate recognition and is currently being utilized in glass tempering systems throughout the world.
U.S. Pat. Nos. 3,934,970 and 3,947,242 each disclose a glass tempering system including a furnace whose roller conveyor has the basic type of friction drive mechanism discussed above but with ends of the conveyor rolls projecting outwardly through side slots defined between upper and lower housings of the furnace. The continuous drive loops of the drive mechanism are driven over external support surfaces to support and frictionally drive the roll ends and thus operate at a much lower temperature than is the case when located within the heating chamber. The lower temperature involved with this external location facilitates the use of continuous chains for the drive loops as opposed to the solid steel belts that are utilized when the drive loops pass through the heating chamber. These chains have teeth that permit positive driving thereof by toothed sheaves associated therewith as opposed to depending on frictional wrap force about the sheaves as with the solid steel belts. Also, the chains can bend to a much smaller radius of curvature than the solid steel belts and thereby permit the use of smaller diameter sheaves.
U.S. Pat. No. 3,994,711 discloses another glass tempering system incorporating the type of friction drive mechanism discussed above but with the conveyor of the furnace driven in an oscillatory manner so as to oscillate a glass sheet or sheets being heated independently of a roller conveyor of the associated quench unit where the glass is subsequently tempered. During an index cycle, the furnace and quench unit conveyor are coupled to provide coordinated conveyance of a heated glass sheet or sheets from the furnace into the quench unit. A first electric motor drive mechanism of this system drives a roller conveyor of a load station as well as the roller conveyor of the furnace while a second electric motor drive mechanism drives a roller conveyor of an unload station as well as the roller conveyor of the quench unit. During oscillatory driving of glass being heated within the furnace and independent oscillation of glass being cooled within the quench unit, the roller conveyors of the load and unload stations are respectively uncoupled from the first and second electric motor drive mechanisms so as to permit loading of glass to be tempered at the load station and unloading of tempered glass at the unload station. The oscillatory driving of the furnace roller conveyor considerably shortens the length of the total system while still conveying the heated glass sufficiently fast to prevent sagging thereof between the spaced rolls of the conveyor.
U.S. Pat. Nos. 4,133,677 and 4,233,053 each disclose a glass sheet roller conveyor having a continuous drive loop mechanism of the type discussed above wherein two sheaves on which each drive loop is received are driven in opposite directions with different torques. Such a drive maintains a taut condition of the driving reach of the drive loop and thereby facilitates maintenance of the rolls driven thereby in proper alignment for conveyance of glass in a single plane.
U.S. Pat. No. 4,341,546 discloses a roll drive mechanism for a horizontal roller conveyor of glass sheet processing equipment wherein first and second continuous drive loops of the mechanism drive the conveyor rolls over associated lengths of the conveyor that are selectively adjustable to provide a desired transition between adjacent conveyor sections driven by the different drive loops. First and second electric motor drives respectively drive the first and second drive loops to provide synchronous or independent glass sheet conveyance over the different lengths of the conveyor. A pair of each of the first and second drive loops are preferably utilized with opposite ends of the conveyor rolls supported thereon and having driven and reduced sized portions that permit end-for-end roll repositioning to adjust the transition between the driving of the loops. The preferred construction of this drive mechanism utilizes chains as the drive loops with the roll ends projecting outwardly through side slots in an associated glass sheet heating furnace which is an application for which the drive mechanism has particular utility in providing conveyor roll driving to convey glass sheets.
With the type of friction drive mechanism involved with the patents discussed above, the drive loops moving over the associated support surfaces are resisted by sliding friction caused by the combined weights of the conveyed glass sheets, the rolls, and the chain itself. Such friction can be considerable with furnaces where the conveyor rolls are made of ceramic which is relatively heavy, especially in long furnaces with a large number of rolls. Also, because of the "stick-slip" phenomenon on long conveyors, the friction can vary at different locations along the length of the conveyor so as to stretch and shorten the drive loop in a manner that can inhibit uniform driving of all of the conveyor rolls at the same speed.